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‘Let the cat out of the bag’ means to accidentally reveal a secret.
Examples:
“I let the cat out of the bag about their party plans.”
Or
“She let the cat out of the bag and finally told her parents about her plans of getting married.”
The Origins
The phrase originates from livestock fraud at markets where animals were sold in sacks, usually piglets. Merchants would sell customers live piglets and, after putting a pig in a sack for easier transport, would sometimes swap the pig for a cat when the customer looked away. The buyer wouldn’t discover they’d been cheated until they got home and literally let the cat out of the bag. The phrase has been around since the 15th century.